TITLE:   GCN CIRCULAR
NUMBER:  21090
SUBJECT: Konus-Wind observation of GRB 170510A
DATE:    17/05/12 14:07:37 GMT
FROM:    Dmitry Svinkin at Ioffe Institute  <svinkin@mail.ioffe.ru>

D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, A. Tsvetkova, A. Lysenko, A.Kozlova, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 170510A
(Fermi-LAT detection: Bissaldi, et al., GCN Circ. 21080;
Fermi-GBM detection: Roberts, et al., GCN Circ. 21085)
triggered Konus-Wind at T0=18750.788 s UT (05:12:30.788).

The burst light curve shows a multipeaked structure
which starts at ~T0-2 s and has a total duration of ~141 s.
The emission is seen up to ~18 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB170510_T18750/

As observed by Konus-Wind, the burst
had a fluence of 5.70(-0.70,+1.51)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+20.688 s,
of 5.60(-1.12,+1.63)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+137.216 s)
is best fit in the 20 keV - 18 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.84(-0.14,+0.17),
the high energy photon index beta = -3.12(-6.88,+0.73),
the peak energy Ep = 261(-36,+42) keV
(chi2 = 76/96 dof).

The spectrum near the maximum count rate
(measured from T0+16.640 to T0+22.528 s)
is best fit in the 20 keV - 18 MeV range
by a power law with exponential cutoff model (CPL):
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.72(-0.09,+0.10)
and Ep = 331(-25,+29) keV (chi2 = 89/92 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.56
(chi2 = 88/91 dof).

In this (and only this) time interval, a prominent count excess
over the Band (and CPL) function is seen in the ~2-18 MeV range,
suggesting a presence of a hard spectral component.
Fitting this spectrum by a CPL + power law (PL) model
yields alpha = -0.48(-0.31,+0.44),
Ep = 301(-31,+50) keV, and
PL photon index of about -1.6 (chi2 = 85/90 dof),
with the component energy flux ratio (PL to CPL) of ~ 0.6
(20 keV - 10 MeV).

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.